A regulatory motif in nonmuscle myosin II-B regulates its role in migratory front–back polarity

In this study, we show that the role of nonmuscle myosin II (NMII)-B in front–back migratory cell polarity is controlled by a short stretch of amino acids containing five serines (1935–1941). This motif resides near the junction between the C terminus helical and nonhelical tail domains. Removal of this motif inhibited NMII-B assembly, whereas its insertion into NMII-A endowed an NMII-B–like ability to generate large actomyosin bundles that determine the rear of the cell. Phosphomimetic mutation of the five serines also inhibited NMII-B assembly, rendering it unable to support front–back polarization. Mass spectrometric analysis showed that several of these serines are phosphorylated in live cells. Single-site mutagenesis showed that serine 1935 is a major regulatory site of NMII-B function. These data reveal a novel regulatory mechanism of NMII in polarized migrating cells by identifying a key molecular determinant that confers NMII isoform functional specificity.